1. Field of the Invention
This invention relates to a process for etherifying phenols. More particularly, this invention relates to catalytically etherifying phenols with low molecular weight alkyl alcohols or ethers in the presence of a catalyst comprising a sulfated composite of one or more transition metal oxides supported on alumina.
2. Background of the Disclosure
The catalytic etherification of alkylation of phenols with low molecular weight alkyl alcohols, such as the methylation of phenol to anisole with methanol is well-known in the art. For example, British Patent No. 600,837 discloses the formation of anisole over an activated alumina catalyst while British Patent No. 1,488,325 shows anisole production over either an eta or gamma alumina catalyst. Inoue and Enomoto teach the alkylation of phenols with methanol over ferric oxide and chromic oxide catalysts (Inoue and Enomoto, ALKYLATION OF PHENOLS ON Fe.sub.2 O.sub.3 and Cr.sub.2 O.sub.3, Chem. Pharm, Bull., 24(9)2199-2203(1976). However, the selectivity and stability of these catalysts is not adequate enough for commercial use. It is well-known that the catalytic methylation of phenol produces coke which rapidly deactivates the catalyst and the catalyst then has to be regenerated. As a practical matter, these prior art catalysts cannot be regenerated due to their instability under regenerating conditions.
A particularly significant application for the etherification of phenols and cresols resides in the synthetic fuels area, such as raw or untreated naphtha fractions or feeds derived from the liquefaction of coal. The aromatic hydroxyl compounds, such as phenol, cresol and their homologues, present in raw coal naphtha contribute to its instability and also tend to poison catalysts used to reform these naphthas to increase their octane value. Before raw coal naphtha can be reformed to increase its octane value, it must be hydrorefined or refined with hydrogen to eliminate sulfur and nitrogen compounds present therein which would otherwise poison the reforming catalyst. If phenols are present in the raw naphtha during the hydrorefining operation, the oxygen present in the phenolic hydroxyl groups results in a hydrogen debit with no significant increase in the octane value of the naptha. On the other hand, the corresponding alkyl and aryl ethers of phenol or phenols, such as anisole, are useful blending agents for improving the octane value of coal-derived naphthas. Therefore, it would be a significant improvement to the art if one could etherify the phenols derived from such naphthas and, more preferably, is such etherification could be accomplished without having first to remove the phenols from the raw naphtha. However, any catalyst useful for etherifying the phenols in or derived from coal liquids must be resistant to poisoning deactivation and must also be capable of being successfully regenerated without appreciable loss of catalytic activity.
Those skilled in the art also know that, the etherification of phenols with alcohols is normally accompanied by the alkylation of the aromatic ring. Although the main product of ring methylation of phenol by methanol is ortho-cresol, the formation of 2,6-xylenol, is also possible. These ring methylated products have all the undesirable properties of phenol with respect to poisoning reforming catalysts, etc. Therefore, a good etherification catalyst should be more selective to oxygen methylation than to ring methylation. This preferred selectivity may be defined as the ratio of the yield of ether products to the yield of all alkylated products. If the process is limited to monomethylation, the selectivity to anisole for the methylation of the parent phenol is defined as the yield of anisole divided by the yield of anisole plus cresols.